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Adhérence de barrières thermiques pour aube de turbine avec couche de liaison β-(Ni,Pt)Al ou β-NiAl(Zr)

Abstract : In order to understand the degradation of thermal barrier coating (TBC) systems better, we determined the adhesion energy (Gc) between the bond coat and the top coat and its evolution during cyclic oxidation at 1100°C. This energy was evaluated by means of a modified four-point bending test for two thermal barrier systems. The systems are constituted of a YPSZ EBPVD ceramic top coat deposited on an AM1 Ni-based superalloy with either a β-(Ni,Pt)Al bond coat or a newly developed Zr-doped β-NiAl bond coat. The results show a similar evolution of the adhesion energy during cyclic oxidation for both systems, indicating that the Zr-doped β-NiAl bond coat constitutes an alternative to the β-(Ni,Pt)Al bond coat.
Observations of the fracture surfaces of the adhesion test samples as well as a microstructure study conducted on oxidised samples revealed that microstructural degradations leading to their spallation are different. The failure of the system including the Pt-modified bond coat was mainly due to interfacial voids formation and to extensive rumpling of the bond coat. In contrast, no interfacial voids were observed in the system including the Zr-doped bond coat and its interface remained rather smooth during its whole lifetime. The high level of residual in-plane compressive stress in the alumina scale of this system, measured by photoluminescence piezospectroscopy, results in the development of high out-of-plane tensile stress at the top of the interfacial undulations. As a consequence, decohesions appear and eventually lead to the delamination.
An energetic model of TBC spallation was used to predict their lifetime. The model is based on the assumption that the TBC failure is induced by the accumulation of strain energy density in the alumina scale and in the top coat and resisted by the interfacial fracture toughness. The consistency and reliability of our analysis are ensured by the evaluation of the available energy to drive the failure and the interfacial fracture toughness from experimental results obtained on the same TBC specimens. This approach provides a reasonable prediction of the lifetime of the TBC systems.
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Submitted on : Sunday, February 24, 2008 - 1:26:40 PM
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Pierre-Yvan Théry. Adhérence de barrières thermiques pour aube de turbine avec couche de liaison β-(Ni,Pt)Al ou β-NiAl(Zr). Mécanique [physics.med-ph]. Université Joseph-Fourier - Grenoble I, 2007. Français. ⟨tel-00258692⟩

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